A magnetic recording medium which is generally and widely used employs as a binder a thermoplastic resin such as a vinyl chloride/vinyl acetate resin, a vinyl chloride/vinylidene chloride resin, a cellulose resin, an acetal resin, a urethane resin, or an acrylonitrile/butadiene resin alone or in combination. However, such a magnetic recording medium has defects in that the magnetic layer has poor wear-out resistance so that the running systems of the magnetic tape are readily stained.
It is also known that a thermosetting resin such as a melamine resin or an urea resin is used as a binder and that a binder is chemically crosslinked by adding an isocyanate compound or a compound having an epoxy ring into the above described thermoplastic resin. However, these crosslinkable binders have defects in that (1) the storage stability of the magnetic particles dispersion is poor and, accordingly, the homogeneity of the magnetic coating composition as well as the homogeneity of the quality of the magnetic recording medium obtained, cannot be assured and (2) a heat treatment process for hardening the coated film is required after being coated and dried, and thus a long period of time for the production process is necessary.
In order to avoid the above defects, a method for preparing a magnetic recording medium has been proposed in which an acrylate type oligomer and monomer which can be hardened by radiation exposure after drying is used as a binder as disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 13639/72, 15104/72, 77433/75 and 25231/81 (the term "OPI" used herein means an unexamined published application). However, a magnetic recording medium having good electromagnetic properties and high durability cannot be obtained by the above-described methods.
Recently, high image quality and high audio quality have been required for magnetic recording medium. In order to attain a high image quality and a high audio quality, it is necessary that a video head or an audio head should come into close contact with the surface of the magnetic layer and, thus, there is a need to improve the surface smoothness of the magnetic recording medium and there should be a remarkable improvement in the dispersibility of the ferromagnetic particles.
On the other hand, friction in the running system of, e.g., a video tape recorder, becomes high, since the surface of the magnetic layer becomes smooth, and thus the running tension becomes high. Under these severe circumstances, extremely high running durability is required for the magnetic recording medium.
However, when a magnetic recording medium runs in the video tape recorder, dust and contaminants adhere on the magnetic recording medium due to the static charge of the medium, causing an increase in drop outs and deteriorating running durability.
Because of the above reasons, a magnetic recording medium having good magnetic layer surface smoothness, good ferromagnetic particle dispersibility, and good running durability has not yet been realized.
In order to avoid the prior art problems, the present inventors have studied the use of thermoplastic resins and thermosetting resins as a binder, the addition of binders which are crosslinkable by the chemical reaction to these resins, and the use of resins which can be hardened by radiation crosslinking, and as a result have attained the present invention.